Cyclohexanol and cyclohexanone can be produced commercially from cyclohexane. The first step in such a process is oxidation of the cyclohexane by an oxygen-containing gas, e.g. air or oxygen-enriched air, to produce cyclohexanol, cyclohexanone and cyclohexyl hydroperoxide (CHHP). The mixture of cyclohexanol (A) and cyclohexanone (K) is commonly referred to as “KA” or “KA oil”. The reaction is generally conducted at temperatures from about 130° C. to about 200° C. Different types of reactors are in commercial use and include single autoclaves, multiple autoclaves in series, horizontal single reactors with multiple compartments, and multistage column reactors. Air is generally used as the source of oxygen. Any unreacted oxygen (along with the nitrogen present in the air) leaves the reactor or reactors as a gaseous effluent. The gaseous effluent also contains vaporized cyclohexane and other compounds. The unreacted oxygen is commonly referred to as “oxygen leakage.” The vaporized cyclohexane and other products in the gaseous effluent are condensed and recovered, and the off-gases leave the system, usually to an abatement system. The KA product is recovered from the liquid effluent from the reactor or reactors, and the unreacted cyclohexane is recycled.
It has been observed that the lower the oxygen leakage from a reactor, the higher the formation of undesirable byproducts and hence the lower is the yield to desirable products. In the oxidation of cyclohexane, the yield of cyclohexanone, cyclohexanol and cyclohexyl hydroperoxide, can be optimized by operating at high oxygen leakage (i.e. concentration of unreacted oxygen in the mixture of cyclohexane free oxygen, nitrogen and other gases and vapors). Unfortunately, at oxygen leakage concentration in excess of 8 vol %, unsafe flammable mixtures can form in the effluent gas stream. Therefore, as a margin of safety the oxygen leakage is usually kept below 4 vol %. Higher oxygen leakage also means that the air being fed to the reactor(s) is not being fully utilized. In other words, the process requires more air, which leads to increased compression cost. In addition, an increased volume of off-gas causes increased cost for off-gas treatment. U.S. Pat. No. 3,957,876 (Rapoport & White) teaches a method to reduce oxygen leakage from a cyclohexane oxidation process through the use of a so-called clean up reaction zone. The Rapoport & White patent discloses a process of cyclohexane oxidation in a column reactor that has a number of perforated trays for contacting an oxygen-containing gas with liquid cyclohexane. The column has two zones. Liquid cyclohexane enters the top part of the top zone, denoted “clean up” zone, and flows downward through the trays in the clean up zone where it contacts the gaseous effluent from the bottom zone in a counter-current fashion. The liquid effluent from the clean up zone comprising liquid cyclohexane, CHHP, K and A enters the top part of the bottom zone and flows downward through the trays in the bottom zone here it contacts an oxygen-containing gas in a counter-current fashion. The oxygen containing gas enters the bottom part of the bottom zone. The bottom zone accomplishes the major part of the oxidation reaction. A liquid effluent comprising cyclohexane, CHHP, K and A is withdrawn from the bottom part of the bottom zone. The clean up zone allows additional consumption of oxygen by reacting it with cyclohexane and thus produces an off-gas that contains oxygen of adequately low concentration so that an explosion hazard can be avoided.
One disadvantage in the Rapoport & White method is that the entire flow of cyclohexane is contacted with the gaseous effluent from the bottom zone. Since the concentration of oxygen is significantly low in the gaseous effluent to be treated, a high reaction temperature, and/or catalyst is required to consume enough oxygen to reduce the concentration of oxygen in the off-gas to an acceptable level. The entire cyclohexane flow, therefore, has to be heated to this high temperature. Since the same hot cyclohexane is used for reaction in the bottom zone, the reaction temperature in the bottom zone is high. It is well known in the art that high reaction temperature in the cyclohexane oxidation process is detrimental to yield to desirable products since high temperature is favorable for producing undesirable byproducts.
It would, therefore, be desirable to have a cyclohexane oxidation process in a column reactor, as taught by Rapoport & White, that would have low oxygen concentration in the off-gas and that would allow a lower reaction temperature in the bottom zone compared to that described in the Rapoport & White patent. It would also be desirable to have processes to accomplish low oxygen concentration in the off-gas, said processes being applicable to other types of reactors used in cyclohexane oxidation, e.g. single autoclaves, multiple autoclaves in series, and horizontal single reactors with multiple compartments.